Mercury in Capricorn is how you think and talk embodied, deliberate, durable.
Capricorn Mercury thinks slowly, concretely, and only speaks when something is solid. Words are tools, not ornaments. People sometimes mistake the slowness for hesitation; it is actually quality control. Your cardinal tends to start things — to initiate, to push the first move, which colours how this Mercury actually shows up day to day.
The shadow: overthinking, gossip, sharp speech weaponized against tenderness. The work is befriending your specific kind of mind instead of fighting it.
Practice: notice the speed your mind likes. Do not borrow another sign's tempo. Read what your tempo reads best. Speak to who actually listens at your speed.